This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to mapping the magnitude of RF fields in a MRI pulse sequence.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field (B0), exciting nuclear spins in the object with a RF magnetic field (B1), and then detecting signals emitted by the excited spins as they precess within the magnetic field (B0). Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
For in vivo MRI at high field (≧3 T) it is essential to consider the homogeneity of the active B1 field (B1+), particularly if surface coils are used for RF transmission. The B1+ field is the transverse, circularly polarized component of B1 that is rotating in the same sense as the magnetization. When exciting or manipulating large collections of spins, nonuniformity in B1+ results in nonuniform treatment of spins. This leads to spatially varying image signal and image contrast and to difficulty in image interpretation and image-based quantification. The B1+ field experienced by spins within the body is influenced by several factors including the distance from the RF transmit coil, tissue dielectric constant, and factors related to the body size and RF wavelength. In high-field (≧3 T) abdominal, cardiac, and neuro imaging, B1+ inhomogeneity on the order of 30-50% has been predicted and observed. When using surface coil transmission, even greater variations in B1+ can be observed over typical imaging FOVs.
There are several existing B1+ mapping methods based on measurements at progressively increasing flip angles, stimulated echoes, or signal ratios. The most simple and straightforward of these methods is the double-angle method, which involves acquiring images with two flip angles α and 2α, where TR>>T1 such that image signal is proportional to sin(α) and sin(2α), respectively. The B1+ field is derived from the ratio of signal magnitudes. Previous double-angle approaches have been limited by the requirement of long TRs and therefore long imaging times and motion compensation issues. While accurate in static body regions, these methods are not practical in areas of the body that experience motion, such as the chest and abdomen.